Reactive fluid compositions, otherwise known as tough acrylic adhesive compositions, are well known in the art as excellent adhesive systems. Typically, these are two-part systems which offer the advantage of rapid cure and high strength along with good shelf life. Curing of these compositions occurs in the presence or absence of air at room temperature, resulting in tough, elastomeric bonds. It is ordinarily not necessry to carefully prepare the surface prior to bonding because these compositions are not particularly sensitive to dirt, grease or other matter which affects surface bondability.
The first main component of the adhesive composition is generally comprised of polymer-in-monomer solutions. Typically, these adhesive compositions are vinyl or acrylic monomers containing chlorosulfonated polyethylene, with a Brookfield viscosity of up to one million. Chlorosulfonated polyethylene is generally the reaction product of a linear or branched polyethylene and sulfonyl chloride and is commercially available under E. I. DuPont De Nemours' & Co. trademark Hypalon. Such adhesive compositions are disclosed in U.S. Pat. Nos. 3,890,407, 3,962,372, 4,106,971 and 4,112,013, which are hereby incorporated by reference.
A mixture or solution of accelerators, promotors, initiators, and free radical generators, all generally defined in the above-mentioned patents as polymerization catalysts, are generally the other main component of the reactive fluid compositions. It is often useful to include one or more of these catalysts in the first part of the composition (monomer portion) as well as the second part.
Examples of typical prior art free radical generators and organic peroxides, hydroperoxides, peresters, persalts and peracids. Amines, such as N,N-dimethylaniline, are ordinarily employed as initiators, and transition metals such as manganese or nickel serve as promoters.
The accelerators of the prior art are generally comprised of amine-aldehyde condensation products. Typically, these accelerators are the products of the reaction between an aliphatic aldehyde (C.sub.1-12) and an aromatic amine (maximum of 18 carbons). U.S. Pat. No. 3,890,407 lists the various amines and aldehydes useful as condensation reactants. The reaction between butyraldehyde and aniline produces a condensation product mixture which is a particularly useful accelerator. A well known commercial accelerator of this specific reaction is marketed by E. I. DuPont De Nemours & Co. under the trademark DuPont 808. The term "accelerator" is commonly used in the art as synonymous with the terms "activator" and "primer." For purposes of this invention, the term "activator" will mean the mixture or solution of products formed by the condensation reaction of an amine and an aldehyde.
The amine-aldehyde condensation reaction is not an addition reaction which results in the high purity yield of simple adduct. Rather, there is a variety of resulting reactions and products which can occur, e.g., see U.S. Pat. Nos. 1,780,334, 1,908,093 and 2,704,750. Research has been conducted on improving the yield of the aldehyde-amine adduct through various mechanisms. For example, see Patrick, "The Reaction of Aldehydes with Amines", Journal of the American Chemical Society," 1952, Vol. 74, pages 2984; and Craig, "N-phenyl-3, 5-diethyl-2-propyl-1, 4-dihydropyridine", Journal of the American Chemical Society, Vol. 70, pg, 1624, 1948; U.S. Pat. No. 2,704,759 to Patrick.
In most instances in these prior art adhesive compositions, the amine-aldehyde solution or mixture is the second part of a two-part composition, the first part being the polymer-in-monomer portion. Usually, the activator portion is applied to one or both surfaces to be joined, prior to application of the first part. It is especially necessary to apply activator to both surfaces to be bonded if there is a large or irregular gap between the surfaces to be bonded. In such cases, the adhesive must be applied to one of the activated surfaces coated with activator before assembly of the parts. Frequently, the geometry or orientation of the assembly parts dictates whether or not the adhesive must be applied over the surface primed with the activator or whether one assembly part can be coated with activator and the other part coated with the adhesive composition.
One present disadvantage to the prior art adhesive systems which use aldehyde-amine condensation products as surface activators is that the adhesive compositions do not readily adhere to the oily surface of the activator. The adhesive tends to slip or "skate" along the surface of the activator and to flow off the bond area prior to assembly. In slip-fit assemblys, e.g., where a rod is to be loosely fitted into a tube, the adhesive is pushed out as the parts are mated. This problem is magnified when the parts to be assembled are positioned or oriented such that gravity accentuates the slippage. Even on a flat surface it is difficult to apply the adhesive onto the activated area, because as the assembly parts are pushed together, the adhesive slips out of the overlapping joint area.
This "skating" of the adhesive off the activated surface is believed to be caused by a variety of substances which prevent proper wetting and interaction of the adhesive with the surface to be bonded. The "active ingredient", e.g., the ingredient in the activator which is believed to react with the polymerizable portion of the adhesive composition, causing an acceleration of polymerization, is thought to be a substituted dihydropyridine compound. For example, in the condensation reaction between butyraldehyde and aniline, the substituted dihydropyridine (identified in Craig, JACS 70 1648 as N-phenyl-3, 5-diethyl-2-propyl-1,4-dihydropyridine) compound is believed to be N-phenyl-3,5-diethyl-2-propyl-1,2-dihydropyridine. Due to the nature of the condensation reaction used to make amine-aldehyde activators, many other compounds are formed which do not enhance the activator's ability to function as an accelerator and which cause the polymerizable portion (polymer-in-monomer part) of the adhesive composition to slide off the surface of an activator-coated part. The result is poor wetting of the surface by the adhesive, resulting in low bond strength. Additionally, these noncontributing products, which in the context of the intended use of the activators can be considered by-products, impart an obnoxious odor and a brown staining color which require ventilation and precautionary handling measures and cause aesthetic imperfections in the surfaces to be bonded, particularly on those surfaces on which the bondline is visible. The polymerizable portion of the adhesive composition is generally a clear, viscous material, but when in contact with the activated surface takes on the brown dirty color of the activator.
It is apparent that a need exists for an activator which has all the advantages of the prior art activators, yet does not cause the skating problems, odor and color problems associated with those activators. It is the intention of the instant invention to overcome those disadvantages, while maintaining the excellent acceleration properties which are commonly possessed by these amine-aldehyde condensation products.